Our objective is to understand the role of the adhesive protein thrombospondin-1 in regulating tumor cell adhesion, growth, motility, and metastasis. As is the case for many adhesive proteins, thrombospondin has binding sites for several matrix components and binds to several types of cell surface receptors. We have shown that thrombospondin promotes adhesion and migration of melanoma and several other tumor cell lines and modulates migration and proliferation of endothelial cells. The latter activities account for the anti-angiogenic acitivty of thrombospondin-1. Using synthetic peptides and recombinant fragments, we have identified three functional sites in thrombospondin-1. Specific peptide sequences containing the motif Trp-Ser-Xaa-Trp were identified that promote cell adhesion and regulate endothelial cell motility and proliferation. The Arg-Phe-Lys sequence in the second type I repeat activates latent TGF-beta, whereas adjacent peptides inhibit its activation by intact thrombospondin-1. Stable analogs of these peptides inhibit tumor growth in animal models of breast cancer and have potential clinical applications in cancer and other diseases associated with abnormal angiogenesis and in regulating wound repair, inflammatory responses, and fibrosis. We are developing experimental approaches to understand the molecular mechanisms of these multiple interactions of cells with thrombospondin and the cellular responses they elicit. We are interested in the direct effects of thrombospondin expression on tumor cells, the role of thrombospondin in neovascularization of tumors, and its role in other tumor cell interactions with endothelium during metastasis.